Result type

In functional programming, a result type is a monadic type holding a returned value or an error code. They provide an elegant way of handling errors, without resorting to exception handling; when a function that may fail returns a result type, the programmer is forced to consider success or failure paths, before getting access to the expected result; this eliminates the possibility of an erroneous programmer assumption.

Examples

• In Elm, it is defined by the standard library as type Result e v = Ok v | Err e.
• In Haskell, by convention the Either type is used for this purpose, which is defined by the standard library as data Either a b = Left a | Right b, where a is the error type and b is the return type.
• In Kotlin, it is defined by the standard library as value class Result<out T>.
• In OCaml, it is defined by the standard library as type ('a, 'b) result = Ok of 'a | Error of 'b type.
• In Rust, it is defined by the standard library as enum Result<T, E> { Ok(T), Err(E) }.
• In Scala, the standard library also defines an Either type, however Scala also has more conventional exception handling.
• In Swift, it is defined by the standard library as @frozen enum Result<Success, Failure> where Failure : Error.
• In C++, it is defined by the standard library as std::expected<T, E>.

Rust

The result object has the methods is_ok() and is_err().

const CAT_FOUND: bool = true;

fn main() {
    let result = pet_cat();
    if result.is_ok() {
        println!("Great, we could pet the cat!");
    } else {
        println!("Oh no, we couldn't pet the cat!");
    }
}

fn pet_cat() -> Result<(), String> {
    if CAT_FOUND {
        Ok(())
    } else {
        Err(String::from("the cat is nowhere to be found"))
    }
}

See also

• Option type
• Exception handling
• Tagged union
• Return type